Support optional AWS SNS module for mobile push notification

[longnd]

Why

For API projects that needs to integrate with mobile app, it is common to send push notification using AWS SNS, e.g. in Worpt API (https://github.com/nimblehq/worpt-infrastructure/tree/main/modules/sns).
To avoid repeating that work, we can introduce SNS as an optional module for the infrastructure template.

Who Benefits?

• The client as the time spent on setting up the infrastructure with AWS SNS will be shorter.
• The development team, as there is less repetitive work between projects.